Laser oscillating device

ABSTRACT

A support unit ( 20   a ) for supporting an OPM holder ( 15   a ) so as to be vertical to the laser beam axis is disposed in the lower part of the OPM holder ( 15   a ). A rotary shaft ( 19 ) is inserted into the support unit ( 20   a ) and rotary shaft support unit ( 20   b ), and the OPM holder ( 15   a ) and DT base ( 17 ) are assembled together. Thus, a rotation support unit ( 200 ) is composed. The rotation support unit has a degree of freedom in the rotating direction of arrow ( 202 ). On the other hand, in the lower part of an RM holder ( 15   b ), a support bar ( 21 ) is provided. At the DT base ( 17 ), a rotating element ( 22 ) and a rotating element support unit ( 23 ) supporting the rotating element are composed so as to support the support bar. Thus, a slider structure ( 220 ) slidable in the optical axis direction is formed. The slider structure has a degree of freedom in the direction of arrow ( 222 ).

TECHNICAL FIELD

The present invention relates to a laser oscillator, and moreparticularly to an axial flow type gas laser oscillator having adischarge tube disposed in the optical axis direction.

BACKGROUND ART

FIG. 25 shows an example of a schematic configuration of a so calledaxial flow type gas laser oscillator. Referring to FIG. 25, the axialflow gas laser oscillator (hereinafter called AFGLO) is explained.

As shown in FIG. 25, the AFGLO is mainly composed of a laser resonator,a power supply unit 4, and a laser gas circulation part.

The laser resonator is composed of a discharge tube 1 having a dischargespace 5, a rear mirror (hereinafter called RM) 6, and an output mirror(hereinafter called OPM) 7. The discharge tube (hereinafter called DT) 1is composed of glass or other dielectric materials, and electrodes 2, 3are disposed near both ends of the DT 1. In the DT 1 between theelectrodes 2 and 3 the discharge space (hereinafter called DA) 5 isplaced. The RM 6 and OPM 7 are disposed to enclose a plurality of DAs 5.The RM 6 is a reflector having a reflectivity of nearly 100%. The OPM 7is a partial reflector, and a laser beam 8 is emitted from the OPM 7.

The power supply unit 4 is connected to the electrodes 2, 3 to perform adischarge in the DA 5.

The laser gas circulation part (hereinafter called LGCP) is composed ofa blower 13, heat exchangers 11, 12, a laser gas passage 10, and aplurality of DAs 5 in DTs 1. The laser gas circulates in the LGCPcomposing the AFGLO in a direction of arrow 9. The blower 13 is forcirculating the laser gas. By this blower 13, the flow velocity of lasergas is set at about 100 m/sec in the DA 5. The pressure in the LGCP isabout 100 to 200 Torr. When a specified voltage is applied to theelectrodes 2, 3 from the power supply unit 4, the DA 5 discharges. Bythis discharge and operation of the blower, the temperature of the lasergas elevates. The heat exchangers 11 and 12 are provided to cool thelaser gas raised in temperature.

Described above is the configuration of the conventional AFGLO, and itsoperation is explained below.

The laser gas sent out from the blower 13 is guided into the DT 1through the laser gas passage 10. In this state, when a specifiedvoltage is applied to the electrodes 2, 3 from the power supply unit 4,the DA 5 discharges. The laser gas in the DA 5 obtains this dischargeenergy and is excited. The excited laser gas becomes a resonant state inthe laser resonator formed of the RM 6 and OPM 7. As a result, a laserbeam 8 is emitted from the OPM 7. The output laser beam 8 is used forlaser machining or other application.

Problems of the conventional AFGLO are described below.

A first problem is described.

FIG. 26 shows a schematic configuration of the laser resonator includingan optical bench of conventional AFGLO. The OPM 7 is held by an outputmirror holder 150 a. The RM 6 is held by a rear mirror holder 150 b. Onthe other hand, the DT 1 is held by a discharge tube holder base(hereinafter called DT base) 170 which is an optical bench, by way of adischarge tube holder (hereinafter called DT holder) 160. Both ends ofthe DT base 170 are connected to corresponding mirror holders 150 a, 150b. The mirror holders 150 a, 150 b, and DT base 170 are assembled to bein an unitary structure. The DT holder 160 and mirror holders 150 a, 150b are connected through a connection tube 180 with both ends held withO-rings or the like so as to be slidable.

In this configuration, the axis linking the center of the RM 6 and thecenter of the OPM 7, and axes of the RM 6 and OPM 7 are disposed to bevertical to each other. That is, the RM 6 and OPM 7 are disposedparallel to each other. The parallelism is adjusted to a precision ofseveral μm or less to each other. The axis linking the centers of the RM6 and OPM 7 is disposed to coincide with the central axis of the DT 1.

To obtain a normal laser output, following conditions are required;

-   -   the parallelism of RM 6 and OPM 7 of 10⁻⁶ radian or less, and    -   the precision of the axis formed by the mirror and the axis        formed by the DT is tens of μm or less.

To maintain this precision, the mirror holders and the DT base areformed in an unitary rigid structure.

In the conventional laser oscillator having such configuration, thefirst problem is explained.

The degree of vacuum in LGCP is about 100 to 200 Torr. On the otherhand, its outside is an atmospheric pressure (760 Torr). Between theinside and outside of the LGCP, a stress due to the pressure difference(hereafter called vacuum force) is applied. Usually, both ends of the DTbase 170 are held by a support structure (not shown). The LGCP is alsoheld by a support structure (not shown). Therefore, in the DT holder 160c in the central part shown in FIG. 25, a downward stress is applied dueto the pressure difference.

The DT base 170 is made of material of high rigidity such as steel so asnot to be bent by the stress due to such pressure difference. Tomaintain the rigidity, the DT base 170 has a considerably largestructure as compared with other parts such as DT 1.

For the pourpose of increasing the rigidity, however, the size islimited. Therefore, by the vacuum force, the DT base 170 may be bent byabout tens of μm. As described above, the DT base 170 and mirror holders150 a, 150 b are assembled in an unitary structure. Accordingly, if theDT base 170 is bent only by tens of μm, the parallelism between themirror holder 150 a and mirror holder 150 b is changed. By this changein parallelism, the laser output may be lowered.

Besides, since the thermal capacity of the DT base 170 is large, ifambient temperature varies, it cannot follow up the temperature changes.Due to change in ambient temperature, a temperature difference may occurin the parts of the DT base 170 (for example, temperature differencebetween upper part and lower part, or temperature difference betweenright side and left side, as shown in FIG. 26). If a temperaturedifference occurs, the DT base 170 is bent due to a thermal expansion ora thermal shrinkage. As a result, the parallelism of RM 6 and OPM 7cannot be maintained. By this change in parallelism, the laser outputmay be lowered. FIG. 27 schematically shows the change in the laseroutput depending on the ambient temperature.

To address this problem, hitherto, the following measures were taken.

As a measure against bending of DT base 170 by vacuum force, forexample, it was attempted to use a canceler for canceling the stress dueto pressure difference in order to keep balance of stress due topressure difference between inside and outside. However, the cancelergenerated an unexpected stress, and produced adverse effects.

On the other hand, as a measure against the expansion and the shrinkagedue to the temperature difference, it was attempted to control the DTbase 170 at a constant temperature. This attempt is intended to passliquid (for example, water) in the DT base 170, and control the liquidtemperature to remain constant. However, the volume of the DT base 170is large in order increase the rigidity. Therefore, the thermal capacityof the DT base 170 becomes larger, and the temperature difference cannotbe completely eliminated.

A second problem of the conventional AFGLO is as follows.

The flow of laser gas in the DT 1 is preferred to be uniform in the gasflow direction as far as possible from entry of gas in the DT 1 untilits exhaust. When the gas flow is uniform, the discharge state isstable. As a result, the efficiency of laser output versus an electricinput to the DA 5 is enhanced (known as a laser oscillation efficiency).Owing to the specific configuration of the AFGLO, the structure iscomplicated if the laser gas lead-in portion is provided coaxially withthe DT 1. Actually, as shown in FIG. 28, the laser gas lead-in portionis generally disposed nearly at right angle to the DT 1. FIG. 28 andFIG. 29 schematically show the gas flow in the DT 1. FIG. 29 is asectional view along line 29—29 in FIG. 28. In this configuration, asshown in FIG. 28, in the DT 1, particularly near the laser gas inlet137, a vortex 136 is likely to occur in the gas flow. By the vortex, thegas flow in the DT is disturbed. As a result, the laser oscillationefficiency cannot be enhanced. FIG. 30 shows the relation betweenelectric input in the DA 5 and laser output.

As proposed in a prior art (Japanese Patent Laid-Open Publication No.7-142787), a chamber is provided for storing the gas temporarily, and itis connected to the laser gas lead-in portion. By eliminating thedirectivity of laser gas entering the laser gas lead-in portion, it isintended to eliminate a non-uniformity of gas flow in the DT. Accordingto the study by the present inventors, the gas flow becomes not uniformwhen feeding gas is led into the DT from the laser lead-in portion, andvortices were formed in various portions. As a result, the laseroscillation efficiency could not be further enhanced by theconfiguration proposed in Japanese Patent Laid-open Publication No.7-142787.

Further, the conventional AFGLO has a third problem.

When the voltage between the electrodes 2, 3 provided around the DT 1reaches a discharge start voltage, discharge starts. At this dischargestart moment, a large rush current flows into the DT 1. When dischargecurrent starts to flow, the impedance of DT drops, and soon atmaintenance voltage of about 20 kV settles. In this state, the currentis stable and a uniform discharge is obtained. However, due to rushcurrent at the discharge start moment, the discharge is disturbedtemporarily. It takes a certain time until the discharge is stabilized.The value of the rush current is proportional to the discharge startvoltage. It is hence important to lower the discharge start voltage inorder to stabilize discharge.

In a prior art, as shown in FIG. 31, an auxiliary electrode 156 isdisposed near the electrode 2 in the DT 1, and the auxiliary electrode156 and the electrode 3 are connected with a high resistance resistor158 of several MΩ. In this case, since the distance between theauxiliary electrode 156 and electrode 3 is too long, if laser gas isionized between the auxiliary electrode 156 and electrode 2, it isalmost recombined before reaching the electrode 3. Therefore, in thisconfiguration, notable effect for decreasing the discharge start voltagecannot be obtained.

FIG. 32 shows another typical example of prior art. Along the outersurface of the DT 1, a conductor 159 is extended from the electrode 2 tothe electrode 3 side, and an auxiliary electrode 156 is attached to theend of the conductor 159 closer to the electrode 3 side. The auxiliaryelectrode 156 is attached to the outer surface of the DT 1 via aninsulating sheet 162 made of a dielectric material. To lower thedischarge start voltage, it was attempted to reduce the thickness of thedielectric materials, but holes were formed in the wall of the DT 1 dueto micro discharge in a course of time.

Thus, in the conventional AFGLO, usually, a mechanism called auxiliaryelectrode was added. It is an attempt to lower the rush current uponstart of discharge by lowering the insulation breakdown voltage in theDT by some mechanism so as to ignite discharge easily. The auxiliaryelectrode itself was a good idea, but none of the prior arts wassatisfactory in the aspects of performance and reliability.

Summing up, in the conventional AFGLO,

1) Stress occurs in the parts of the resonator due to pressuredifference between the LGCP and the outside at atmospheric pressure. Bythis stress, the DT base 170 may be bent by about tens of μm. Since theDT base 170 and a pair of mirror holders 150 are in unitary structure,if the DT base 170 is bent only by tens of μm, the mutual angle of thepair of mirror holders 150 a and 150 b is varied. As a result, it wasdifficult to enhance the stability of laser output further.

2) In the DT, the laser gas flow tends to be not uniform in the centralpart or in peripheral part of the DT. As a result, uniform gas flow isnot realized. Hence, the energy efficiency could not be enhancedfurther.

3) A large rush current flows in the DT at the discharge start momentwhen the voltage between the electrodes 2, 3 reaches the discharge startvoltage. When the rush current flows at the discharge start moment, alarge current flows, and the discharge is disturbed temporarily.Accordingly, it takes some time until discharge is stabilized, anddischarge is unstable in this period (that is, the laser output isunstable). This transient unstable period of discharge cannot beshortened.

DISCLOSURE OF THE INVENTION

The present invention is made to inprove the above problems.

To address the first problem, the AFGLO of the invention comprises:

-   -   a) a DT for exciting laser medium disposed inside by applying        energy;    -   b) at least a pair of mirrors disposed on an optical axis of        laser beam emitted from the laser medium excited by the DT;    -   c) at least a pair of mirror holders for holding the mirrors;    -   d) a plurality of mirror holder connecting rods for connecting        the mirror holders;    -   e) a DT holder for holding the DT; and    -   f) a DT base for holding the DT holder, wherein,    -   g) one mirror holder is fixed to the DT base in the laser beam        axial direction and in the vertical direction to the laser beam        axis,    -   h) the mirror holder has a degree of freedom in the rotating        direction within the plane including the laser beam axial        direction, and    -   i) the other mirror holder is fixed to the DT base in the        vertical direction to the laser beam axis, and is slidable in        the laser beam axial direction.

To address the second problem, the AFGLO of the invention comprises:

-   -   a) a DT for passing laser gas inside and exciting the laser gas;    -   b) electrodes disposed near both ends of the DT:    -   c) at least one of the electrodes being disposed near the laser        gas inlet of the DT; and    -   d) a laser gas passage for supplying laser gas to the DT,        wherein,    -   e) the configuration suffices the following relation,        1.1 A<B<1.7A    -   where A is the inner diameter of the DT, B is the width in the        gas flow direction of the laser gas passage near the laser gas        inlet of the DT and in the vertical direction to the gas flow        direction in the DT.

To addres the third problem, the AFGLO of the invention comprises:

-   -   a) a DT;    -   b) electrodes disposed near both ends of the DT; and    -   c) a high voltage power supply for applying a high voltage        between the electrodes, wherein,    -   d) a hole is opened in the DT,    -   e) an auxiliary electrode is disposed in the hole,    -   f) the auxiliary electrode is connected to one of the electrodes        via a high resistance resistor,    -   g) the position of the hole opened in the DT is at a distance of        0.4L to 0.7L from the electrode not connected to the auxiliary        electrode where L is the distance between the two electrodes,        and    -   h) the resistance of the high resistance resistor is in a range        of 1 MΩ or more and 100 MΩ or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic configuration of an axial flow gas laseroscillator according to example 1 of the present invention.

FIG. 2 shows a configuration of resonator section of the laseroscillator shown in FIG. 1.

FIG. 3A is a left side view of the resonator section shown in FIG. 2.

FIG. 3B is a right side view of the resonator section shown in FIG. 2.

FIG. 4 is a three-side view showing a coupling area of OPM holder and DTbase of laser oscillator of other configuration of example 1.

FIG. 5A is a detailed view of an optical bench of still another laseroscillator configuration of example 1.

FIG. 5B is an enlarged sectional view of a stand near pillow ballstructure shown in FIG. 5A.

FIG. 6 is a view of RM 6 seen from 6—6 plane shown in FIG. 5A.

FIG. 7 shows difference in laser output due to change in ambienttemperature between an example of the present invention and a prior art.

FIG. 8 shows a configuration of laser oscillator according to example 2of the present invention.

FIG. 9 is a schematic diagram showing laser gas flow in DT and laser gaspassage of laser oscillator in the example of the present invention.

FIG. 10 schematically shows laser gas flow in 10⁻¹⁰ plane shown in FIG.9.

FIG. 11 shows a correlation of width B near laser gas inlet of DT andlaser output.

FIG. 12 schematically shows laser gas flow in DT and laser gas passage.

FIG. 13 is a schematic diagram showing laser gas flow in 13—13 planeshown in FIG. 12.

FIG. 14 shows a correlation of height C from the center of DT, in thecolumnar protrusion provided at the confronting part of the laser gasinlet of DT, and laser output.

FIG. 15 shows a correlation of inside diameter D of the columnarprotrusion provided at the confronting part of the laser gas inlet ofDT, and laser output.

FIG. 16 is a schematic diagram showing laser gas flow near DT and in DT.

FIG. 17 schematically shows laser gas flow in 16—16 plane shown in FIG.16.

FIG. 18 is an overlapped diagram of correlation of width B and laseroutput shown in FIG. 11 with the laser output in the configuration inFIG. 16.

FIG. 19 shows a difference of laser output from electric input to DT,between the example of the present invention and a prior art.

FIG. 20 shows a laser oscillator according to example 3 of the presentinvention.

FIG. 21 schematically shows a detailed configuration of DT section inthe laser oscillator shown in FIG. 20.

FIG. 22 shows the relation between the distance of an auxiliaryelectrode and an electrode to which the auxiliary electrode is notconnected, and the discharge start voltage in example 3 of the presentinvention.

FIG. 23 shows the relation between the resistance of the high resistanceresistor coupling the auxiliary electrode and an electrode, thedischarge start voltage and laser output in example 3 of the presentinvention.

FIG. 24 shows the difference of laser output in example 3 of the presentinvention and a prior art.

FIG. 25 shows a schematic configuration of conventional axial flow gaslaser oscillator.

FIG. 26 is a schematic view of optical bench section of the conventionallaser oscillator.

FIG. 27 shows output stability in the conventional laser oscillator.

FIG. 28 is a schematic diagram showing detail of DT section in theconfiguration of the conventional laser oscillator and laser gas flow.

FIG. 29 is a schematic diagram showing laser gas flow in 29—29 planeshown in FIG. 28.

FIG. 30 is a diagram showing the relation between electric input andlaser output in prior art.

FIG. 31 is a schematic diagram showing configuration of DT section inprior art.

FIG. 32 is a schematic diagram showing configuration of other DT sectionin prior art.

FIG. 33 is a diagram showing the relation between electric input andlaser output in prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Embodiments of the invention are described below with referrence to theaccompanying drawings. FIG. 1 shows a laser oscillator according toembodiment 1 of the invention. FIG. 2 shows a configuration of resonatorsection of the laser oscillator shown in FIG. 1. FIG. 3A is a left sideview of the resonator section shown in FIG. 2. FIG. 3B is a right sideview of the resonator section shown in FIG. 2. Same parts as in theconventional laser oscillator shown in FIG. 25 are identified with samereference numerals, and their description is omitted.

The embodiment is explained by referring to FIG. 1, FIG. 2, FIG. 3A, andFIG. 3B.

An OPM holder 15 a and an RM holder 15 b are supported to be parallel toeach other by a plurality of mirror holder connecting roads (hereinaftercalled MHCR) 14. A rotation support unit 200 is configured to supportthe OPM holder 15 a on a DT base 17. A support unit 20 a for supportingthe OPM holder 15 a to be vertical to the laser beam axis is disposed inthe lower part of the OPM holder 15 a. A rotary shaft support unit 20 bis disposed on the DT base 17. A hole for inserting a rotary shaft 19 isdisposed in the support unit 20 a and rotary shaft support unit 20 b.The rotary shaft 19 is inserted into the support unit 20 a and rotaryshaft support unit 20 b, so that the OPM holder 15 a and DT base 17 areassembled together. The contact portions of the rotary shaft 19 androtary shaft support units 20 a, 20 b are finished to a smooth surfaceso as to rotate smoothly with minimum friction. Or a component extremelysmall in friction against rotation such as ball bearing (or rollerbearing) is inserted. Thus, the rotary shaft 19, support unit 20 a, androtary shaft support unit 29 b are combined to compose the rotationsupport unit 200 for supporting the OPM holder 15 a on the DT base 17.The rotation support unit 200 has a degree of freedom in the rotatingdirection shown by an arrow 202 shown in FIG. 1 and FIG. 2.

On the other hand, a support bar 21 is provided in the lower part of theRM holder 15 b. At the DT base 17 side, a rotating element 22 and arotating element support unit 23 for supporting the rotating element 22are disposed for supporting the support bar 21. In this manner, a sliderstructure 220 slidable in the optical axis direction is formed. Thisslider structure 220 has a degree of freedom in the optical axisdirection shown by an arrow 222 in FIG. 1 and FIG. 2.

In this configuration, the OPM holder 15 a and DT base 17 are fixed inthe vertical direction to the laser beam axial direction. However, theOPM holder 15 a and DT base 17 have a degree of freedom only in therotating direction within the plane including the laser beam axialdirection. As a result, the OPM side mirror holder 15 a and DT base 17can be coupled without deviation of optical axis.

On the other hand, the rear mirror holder 15 b and DT base 17 are fixedin the vertical direction to the laser beam axial direction (however,they are free in the upward direction, strictly). That is, by a weight(own weight) of the mirror holder 15 b, the rear mirror holder 15 b andDT base 17 are supposed to be fixed. Of course, this configuration isfree in the sliding direction of optical axis direction and in therotating direction within the plane including the optical axisdirection. As a result, the RM side mirror holder and DT support unitare also coupled without deviation of optical axis same as at the OPMside.

Folowing explanation is made when the DT base 17 is deformed by thevacuum force or the temperature change. When the DT base 17 is bent byvacuum force, rotation within the plane including the optical axisdirection occurs in the coupling portion of the mirror holder and DTsupport unit. As described above, this portion is free in the rotatingdirection both at the OPM side and RM side. Accordingly, the mirrorholder is free from any force causing change in the parallelism due tovacuum force or thermal stress. In the event of thermal expansion orthermal shrinkage of DT support unit, linear displacement in the opticalaxis direction occurs at the coupling portion with the mirror holder,but since the RM holder is held free in this direction, force to causechange in parallelism is not applied to the mirror holder by the vacuumforce or the thermal stress.

An excellent feature of the configuration of the present invention liesin the coupling portion of the OPM holder and DT support unit. Thissystem also has a degree of freedom structurally other than the rotatingdirection within the plane including the optical axis direction. Forexample, it may be configured to have coupling member high in degree offreedom such as pillow balls at two positions each in the lower part ofthe OPM holder 15 a and RM holder 15 b. In this system, however, thedegree of freedom is limited by fixing at two points. Accordingly, it islikely to cause change in the parallelism due to vacuum force. Besides,the distance between two fixing points varies due to thermal expansionor thermal shrinkage of the mirror holder itself, and hence a force tocause change in parallelism is likely to occur.

FIGS. 4A-4C show detailed views of junction of OPM holder and DT supportunit of a laser oscillator of another configuration of the presentembodiment. The rotary shaft 19, support unit 20 a, and rotary shaftsupport unit 20 b are combined without clearance (without gap). However,if completely free from gap, rotation is not smooth due to friction. Asmentioned above, by inserting a ball bearing in the contact portions ofthe rotary shaft 19, support unit 20 a, and rotary shaft support unit 20b, it is substantially free from clearance to the parallel direction tothe optical axis direction. Besides, as being pushed down by the ownweight of the mirror holder, the central axis of the discharge tube andoptical axes of the mirrors, and a relative position between the mirrorsare hardly changed and are stable. However, concerning the directionvertical to the optical axis, a gap is needed for smooth rotationbetween the support unit 20 a and rotary shaft support unit 20 b. Toprevent looseness due to this gap, an elastic force is employed to pushthe upper rotary shaft support unit 20 a to the lower rotary shaftsupport unit 20 b at one side. FIGS. 4A-4C show an example of suchstructure. For example, two spring members 24 are symmetrically disposedon both sides of the rotary shaft, and the elastic force of the springmembers 24 is applied to the support unit 20 a. Between a spring holder25 provided on the rotary shaft 19 and the upper rotary shaft member 20a, the spring members 24 are placed compressed. Besides, in order thatthe spring holder 25 and spring members 24 may not impede the motion inthe rotating direction, a rotating element such as pillow ball 26 isinserted in the junction between the rotary shaft 19 and spring holder25.

However, as for linkage of RM holder 15 b and DT base 17, it is notlimitted to one-point fixing as described above in the presentembodiment. The RM holder and DT support unit may be fixed at two pointsby using pillow balls or other coupling members having high degree offreedom. At this time, there is no problem if the OPM side is fixed atone point as shown in the embodiment of the present invention.

FIGS. 5A and 5B are detailed views of an optical bench of laseroscillator showing still another configuration of the presentembodiment.

Same as in the embodiment shown in FIG. 2, the mirror holders 15 a and15 b are mutually coupled by means of a plurality of MHCRs 14.Similarly, the OPM holder 15 a is supported on the DT base 17 by meansof the rotation support unit 200. On the other hand, in the lower partof the RM holder 15 b, pillow balls 26 are disposed at two points in thehorizontal direction. The rear mirror holder 15 b is coupled to the DTbase 17 through the pillow balls 26. FIG. 6 is a view of RM 6 seen in adirection from 6—6 shown in FIG. 5. As shown in FIG. 6, for connectingthe DT holder 16, DT base 17, and MHCRs 14, ribs 27 are disposed at fourpositions. The ribs 27 are disposed near the center of MHCRs 14. Theribs 27 and MHCRs 14 are designed to slide slightly in the verticaldirection.

The optical bench stabilizing effect by inserting the ribs 27 isexplained. When the DT base 17 id deformed by vacuum force ortemperature change, in particular, when expanded or shrunk bytemperature change, the following effects occur. At this time, in thecoupling portion of the mirror holder and DT base 17, a lineardisplacement occurs in the optical axis direction. However, the pillowballs are free to move because sliding occurs between the pillow ballsand the shaft passing through the pillow balls. Accordingly, the RMholder is free in this direction. Therefore, any force causing change inparallelism does not occur on the mirror holder due to thermal stress.However, even if a friction is reduced so as to be freely movablestructurally, actually, the friction is not zero. The junction of therear mirror holder 15 b and DT base 17 is pushed in the direction ofgravity (that is, in the downward direction in FIG. 5) due to the ownweight of the rear mirror holder 15 b. Therefore, a frictional forceoccurs in this portion, strictly speaking. On the MHCR 14, too, atensile or compressive force works in the optical axis direction. TheMHCR 14 is a circular column of about 50 mm in diameter, and 1000 to2000 mm in length. When tensile or compressive force works in theoptical axis direction (that is, in the longitudinal direction of MHCR14), the MHCR 14 deflects. At this time, if ribs 27 are not disposed,each MHCR 14 is bent in an arbitrary direction. As a result, theparallelism of the OPM holder 15 a and RM holder 15 b is broken.

When four MHCRs 14 and ribs 27 for coupling the DT holder 16 and DT base17 are disposed, the rigidity of the MHCRs 14 is enhanced, and they arenot bent by frictional force. Therefore, the parallelism of the mutualmirror holders is maintained.

Further, when all MHCRs 14 are made to deflect in the central directionor in the peripheral direction, the parallelism of the OPM holder 15 aand RM holder 15 b is maintained more accurately.

For example, four MHCRs 14 are mutually attracted in the centraldirection by several millimeters by the ribs 27. The mirror holderconnecting members are attracted in the central direction by aboutseveral millimeters by the ribs 27, and are slightly warped. In thisstate, when a tensile or compressive force is applied to the MHCRs 14,all of four MHCRs 14 are deflected in the central direction. As aresult, the parallelism of the OPM holder 15 a and RM holder 15 b ismaintained.

On the contrary, same effects are obtained by warping the four MHCRs 14in mutually opposite directions by the ribs 27.

Thus, in the embodiment of the present invention, the optical benchbecomes very stable, and the effect of keeping parallelism betweenmirrors is extremely large. As a result, always stable laser oscillationis possible, and the laser output is substantially stabilized.

FIG. 7 shows difference in laser output due to change in ambienttemperature between the embodiment of the present invention and a priorart. The axis of abscissa denotes the ambient temperature, and the axisof ordinate represents the laser output. Both in the present embodimentof the invention and in the prior art, it is adjusted so that themirrors may be parallel to each other at an ambient temperature of 20°C. The diagram shows changes of laser output when the ambienttemperature is lowered or raised from this state. As shown in FIG. 7, inthe embodiment of the present invention, as compared with the prior art,the laser output is stabilized substantially in spite of the change inambient temperature. Similar effects are obtained regarding the vacuumforce or the external force.

The present invention therefore provides a laser oscillator having astable optical bench, or a stabile mirror parallelism regardless ofvacuum force, external force or ambient temperature change, and iscapable of obtaining stable laser output all the time.

The connecting rods of the mirror holder connecting rods may be composedof pipes. The connecting rods or pipes are preferably made of materialssmall in coefficient of thermal expansion, so that a small difference inexpansion due to temperature difference is effective for the resonatorof the present invention.

Embodiment 2

Embodiment 2 of the present invention is explained with referrence tothe drawings.

FIG. 8 shows a configuration of laser oscillator according to the secondembodiment of the present invention. FIG. 9 is a schematic diagramshowing laser gas flow of in DT and laser gas passage of laseroscillator in the embodiment of the present invention. FIG. 10 is aschematic diagram showing laser gas flow in 10—10 plane in FIG. 9. Awidth in the gas flow direction near the laser gas inlet 37 of the DT 1,and a width in the vertical direction to the gas flow direction in theDT is defined as B. The inside diameter of the DT is defined as A. FIG.9 shows the laser gas flow in the DT and laser gas passage when arelation between A and B is configured as1.1 A<B<1.7A.In FIG. 9, the laser gas flowing through the laser gas passage of widthB in direction of arrows 9 b is guided into portion of the width B nearthe inlet of the DT. From this portion, laser gas flow is narrowed tothe inside diameter A of the DT. Afterwards, the laser gas flows in adirection of arrows 9 a in the DT. At this time, the laser gas flows tothe downstream side of the DT 1 with a moderate slope from the wideportion (that is, width B) near the inlet of DT 1. Accordingly, the gasflow forms a mild streamline from the inlet 37 of the DT 1 to thedownstream side (thereby not forming vortex). Distribution of laser gasflow in the DT 1 is formed almost uniformly on the whole. At this time,if the width B is smaller than 1.1A (that is, in the prior artconfiguration), vortex is formed in the DT inlet. If the width B islarger than 1.7A, vortex is also formed in the DT inlet. By this vortex,the laser gas flow in the DT is disturbed. FIG. 11 shows a correlationbetween the width B near laser gas inlet of DT and laser output. FIG. 11shows when the width B near the laser gas inlet is in a range of1.1 A<B<1.7A,the laser output is maximum. As the discharge is stabilized in thisrange, the laser output becomes maximum.

FIG. 12 shows a shape of another DT. In the confronting part of thelaser gas inlet of the DT, a circular columnar protrusion of height of Cfrom the center of DT and inside diameter of D is provided. When aninner diameter of the DT to be A, C and D are configured to satisfy thefollowing relations.0.5A<C<0.9A0.7A<D<0.9AFIG. 12 schematically shows laser gas flow in DT and laser gas passage.FIG. 13 is a schematic diagram showing laser gas flow in 13—13 plane inFIG. 12. The laser gas flowing the laser gas passage in a direction ofarrows 9 b is led in from the laser gas inlet of the DT. It furthercollides to the circular columnar protrusion provided in the confrontingpart of the laser gas inlet of the DT. The laser gas further flowsdownstream. Therefore, a mild streamline is formed from the DT inlet tothe downstream side. As a result, the laser gas flow distribution in theDT is formed uniformly on the whole.

On the other hand, if the circular columnar protrusion is too large,vortex is formed in the upper part of the DT, and the streamline in theDT is disturbed. If vortex is formed in various parts of the DT, thelaser gas flow distribution in the DT becomes extremely non-uniform. Asa result, discharge is disturbed, and stable laser oscillation is notmaintained. FIG. 14 shows a correlation between height C from the centerof DT in the circular columnar protrusion provided at the confrontingpart of the laser gas inlet of DT, and laser output.

As shown in FIG. 14, the laser output is maximum when the height C is ina range of0.5A<C<0.9A.As the discharge is stabilized in this range, the laser output becomesmaximum.

FIG. 15 shows a correlation between inner diameter D of the columnarprotrusion provided at the confronting part of the laser gas inlet ofDT, and laser output.

As shown in FIG. 15, the laser output is maximum when the inner diameterD is in a range of0.7A<D<0.9A.As the discharge is stabilized in this range, the laser output becomesmaximum.

Electrodes are provided near the laser gas inlet of the DT. If thecolumnar protrusion near the confronting part is made of metal orconductor, the electric field is disturbed, and discharge is likely tobe disturbed. Therefore, the confronting part must be made of dielectricmaterial same as the DT. Specifically, same as the DT, it is preferablymade of Pyrex, quartz, ceramic, or other dielectric materials.

FIG. 16 is a schematic diagram showing laser gas flow near DT and in DT.FIG. 17 is a schematic diagram showing laser gas flow in 16—16 plane inFIG. 16.

The width in the vertical direction to the gas flow direction near thelaser gas inlet 37 of the DT is defined as B. In the laser gas inletconfronting part of the DT, a circular columnar protrusion 38 of heightof C from the center of the DT and inner diameter of D is provided.

When the inner diameter of the DT 1 is defined as A, it is configured toestablish the relations of1.1A<B<1.7A0.5A<C<0.9A0.7A<D<0.9AThe columnar protrusion 38 provided in the laser gas inlet confrontingpart of the DT is made of ceramic or other dielectric material.

The laser gas flowing direction in the DT is supposed to be 9 a, and thelaser gas flowing direction in the laser gas passage is 9 b.

FIG. 16 shows a combined configuration of the configurations shown inFIG. 9 and in FIG. 12. In this configuration, discharge is furtherstabilized owing to synergistic effects of FIG. 9 and FIG. 12, and theconfiguration is very effective. FIG. 18 is an overlapped diagram ofcorrelation between width B and laser output in the configurations shownin FIG. 11 and in FIG. 16.

FIG. 19 shows a difference of laser output vs electric input to DT,between in the embodiment of the present invention and in a prior art.The discharge electric input is shown on the axis of abscissas, and thelaser output is given on the left side of the axis of ordinate. As shownin FIG. 19, in the embodiment of the present invention, the laser outputis increased substantially as compared with the prior art owing to thelaser gas flow improving effect.

By forming a uniform laser gas flow in the DT, the present inventionprovides a laser oscillator with a substantially enhanced laseroscillation efficiency and an increased laser output.

Embodiment 3

Embodiment 3 of the present invention is explained with referrence tothe drawings. FIG. 20 shows a laser oscillator according to the thirdembodiment of the present invention. FIG. 21 is a schematic diagramshowing a detailed configuration of DT section in the laser oscillatorshown in FIG. 20.

A hole 55 is opened in the wall of a DT 1 evacuated to about 100 to 200Torr. To cap this hole, an auxiliary electrode 56 made of copper,tungsten or the like conductor is provided. The junction area of theauxiliary electrode 56 and DT 1 is sealed with O-ring or other vacuumseal 57. The auxiliary electrode 56 is configured to contact directlywith the laser gas in the DT 1. The auxiliary electrode 56 is connectedto the electrode 3 via a high resistance resistor 58 of several MΩ.Electrode 2 and electrode 3 are connected to the power supply 4.

The operation of this configuration is explained. The auxiliaryelectrode 56 is connected to the electrode 3 by way of the highresistance resistor 58. Accordingly, while current is not flowing in theDT 1, the electrode 3 and auxiliary electrode 56 are at a samepotential.

When the voltage between the electrode 2 and electrode 3 is graduallyincreased by the high voltage power supply 4, the voltage between theelectrode 2 and auxiliary electrode 56 is also increased at the sametime. In the absence of the auxiliary electrode 56, the discharge startvoltage between the electrode 2 and electrode 3 reaches to about 40 kV.However, since the auxiliary electrode 56 is located near the electrode2, the discharge start voltage of the electrode 2 and auxiliaryelectrode 56 is about 23 to 24 kV. That is, when the potentialdifference between the electrode 2 and electrode 3 reaches about 23 to24 kV, discharge starts between the electrode 2 and auxiliary electrode56 with a same potential difference. The laser gas in this dischargepassage (discharge space 5) is ionized. The ionized laser gas flows tothe electrode 3 as indicated by a flow direction 9 of laser gas. By thisionized laser gas, the impedance in the DT 1 decreases, and dischargestarts in the DA 5 between the electrode 2 and electrode 3. On the otherhand, the discharge current between the electrode 2 and auxiliaryelectrode 56 is suppressed by the high resistance resistor 58 of severalMΩ provided between the auxiliary electrode 56 and electrode 3. As aresult, after start of discharge, current hardly flows between theauxiliary electrode 56 and electrode 3.

By this operation, the discharge start voltage can be decreased from theconventional level of 40 kV to 23 to 24 kV.

Thus, the rush current at the discharge start moment can be suppressed,and a stable discharge is obtained.

As described above, in the prior art shown in FIG. 31, the auxiliaryelectrode 156 was disposed near the electrode 2 in the DT 1, and theauxiliary electrode 156 and the electrode 3 were connected through ahigh resistance resistor of several MΩ. In this case, since the distancebetween the auxiliary electrode and the negative electrode is too long,ionized gas is mostly recombined before reaching the negative electrode,even if the laser gas is ionized before, and notable effect was notobtained.

In the present invention, by contrast, when the distance between the twoelectrodes is defined as L, the auxiliary electrode is disposed at aposition of 0.4L to 0.7L from an electrode to which the auxiliaryelectrode is not connected. FIG. 22 shows the relation between thedistance of auxiliary electrode and the electrode to which the auxiliaryelectrode is not connected, and the discharge start voltage. As FIG. 22shows, when the distance is shorter than 0.4L, the ionized laser gasrecombines, and the effect of reducing the discharge start voltage isnot obtained. If the distance is longer than 0.7L, on the other hand,since the distance between the positive electrode and the auxiliaryelectrode is too long, the discharge start voltage climbs up.Accordingly, an appropriate distance between the auxiliary electrode andthe electrode to which the auxiliary electrode is not connected is in arange of 0.4L to 0.7L. FIG. 32 shows another representative example ofthe prior art. Along the outer surface of the DT, a conductor 159 isextended from the electrode 2 to the electrode 3. An auxiliary electrode156 is provided at the end of the conductor 159 closer to the electrode3. The auxiliary electrode 156 is bonded to the wall of the DT 1 via aninsulating sheet 162 made of a dielectric material. The auxiliaryelectrode 156 and electrode 3 form a capacitive coupling by via thedielectrics. This configuration was attempted to ionize a laser gasexisting in the path of current, and decrease the discharge startvoltage. To increase the effect of lowering the discharge start voltage,it was also attempted to reduce the thickness of the dielectrics, butholes were formed in the wall of the DT during a long time operation dueto a corona discharge. In the present invention, since the hole 55 isopened in the DT 1 in the fitting area of the auxiliary electrode 56 forpassing small current upon start of discharge, there is no problem offormation of holes during long time operation, and it is also excellentin long-term reliability.

FIG. 23 shows the relation between the resistance of the high resistanceresistor coupling between the auxiliary electrode 56 and the electrode,and the discharge start voltage and laser output. If the resistance ofthe high resistance resistor 58 is less than 1 MΩ, too much currentflows in the auxiliary electrode, and the discharge in the DA 5 isdisturbed. As a result, high laser output is not obtained. On the otherhand, if the resistance is larger than 100 MΩ, the effect of theauxiliary electrode is small, and the effect of decreasing the dischargestart voltage is not obtained. Besides, the discharge is disturbed byrush current, and the effect of increasing the laser output is notobtained. Therefore, the resistance of the high resistance resistorshould be in a range of 1 MΩ or more to 100 MΩ or less.

Thus, in the prior art, the auxiliary electrode had problems inperformance and reliability, but in the present invention, the laseroutput is stabilized by substantial decrease of discharge start voltage,and the long time reliability is obtained.

FIG. 24 shows the discharge start voltage vs electric input to the DTand laser output in the embodiment of the present invention and these ofthe prior art, in which the axis of abscissas denotes the dischargeelectric input and the axis of ordinate represents the laser output. Asshown in FIG. 24, as the electric input to the DT is increased, theeffect of the embodiment of the present invention is more clearlyexpressed. By stabilizing the discharge by a decrease of the dischargestart voltage, the laser output is increased substantially as comparedwith the prior art.

According to the present invention, the discharge is stabilized bysubstantial decrease of discharge start voltage, and a laser oscillatorwith an outstanding increase in laser output is provided.

INDUSTRIAL APPLICABILITY

The present invention provides a laser oscillator with a stabile opticalbench, or a stabilized mirror parallelism, against vacuum force,external force or ambient temperature change, and a stable laser outputall the time.

By forming a uniform laser gas flow in the DT, the present inventionprovides a laser oscillator with a substantially improved laseroscillation efficiency and an increased laser output.

The present invention stabilizes discharge by significantly decreasingthe discharge start voltage, and hence a laser oscillator with a notablyincreased laser output is obtained.

1. A laser oscillator comprising: a discharge tube operable to passlaser gas inside thereof and to excite the laser gas; a first electrodedisposed at a first end of said discharge tube; a second electrodedisposed at a second end of said discharge tube; and a laser gas passageoperable to supply the laser gas to said discharge tube, said laser gaspassage being connected to said discharge tube, wherein a width B ofsaid discharge tube in a direction normal to a gas flow direction insaid laser gas passage near a connection portion of said discharge tubeand said laser gas passage is larger than an inner diameter A of saiddischarge tube, and a following relation is satisfied1.1A<B<1.7A wherein A>0 and B>0, wherein said discharge tube includes alaser gas inlet, and wherein one of said first electrode and said secondelectrode is disposed near said laser gas inlet.
 2. The laser oscillatorof claim 1, further comprising: a columnar protrusion being provided tosaid discharge tube at a portion opposite to a connection portion ofsaid discharge tube and said laser passage, wherein the followingrelations are satisfied1.1A<B<1.7A0.5A<C<0.9A0.7A<D<0.9A, and wherein C is a height of said columnar protrusions froma center of said discharge tube, and D is an inner diameter of saidcolumnar protrusion.
 3. A laser oscillator comprising: a discharge tubeoperable to pass laser gas inside thereof and to excite the laser gas;and a laser gas passage operable to supply laser gas to said dischargetube, said laser gas passage being connected to said discharge tube,wherein a columnar protrusion is provided to said discharge tube, saidcolumnar protrusion being provided at a portion opposite to a connectionportion of said discharge tube and said laser gas passage, wherein thefollowing relations are satisfied0.5A<C<0.9A0.7A<D<0.9A, and wherein A>0 and is an inner diameter of said dischargetube, C>0 and is a height of said columnar protrusion from a center ofsaid discharge tube and D>0 and is a inner diameter of sad columnarprotrusion.
 4. The laser oscillator of claim 3, wherein said columnarprotrusion is composed of dielectric materials.
 5. The laser oscillatorof claim 2, wherein said columnar protrusion is composed of dielectricmaterials.
 6. A laser oscillator comprising: a discharge tube having twoends and being operable to pass laser gas inside hereof and to excitethe laser gas, said discharge tube being provided with a hole opened toan outside thereof; a laser gas passage operable to supply laser gas tosaid discharge tube, said laser gas passage being connected to saiddischarge tube; electrode disposed at both ends of said discharge tube;a high voltage power supply operable to apply a high voltage betweensaid electrodes; and an auxiliary electrode covering the opened hole,said auxiliary electrodes being provided outside of said discharge tube,wherein said a electrode is connected to one of said electrodes via ahigh resistance resistor, and a distance between the hole and anelectrode not connected with said auxiliary electrode is between 0.4Land 0.7L, where L is a distance between said electrodes disposed at bothends of said discharge tube, and wherein a resistance of said highresistance resistor is 1 MΩ or more and 100 MΩ or less.